Europa, Ganymede, and Callisto: New radar results from Arecibo and Goldstone

Observations of the icy Galilean satellites, conducted during 1987-1991 with the Arecibo 13-cm system and the Goldstone 3.5-cm system, yield significant improvements in our knowledge of the satellites' radar properties. Hardly any wavelength dependence is seen for either the total power radar albedo σ̂_T or the circular polarization ration μ_c. For Europa, Ganymede, and Callisto our 13-cm estimates of mean values and rms dispersions are σ̂_T=2.60+/-0.22, 1.39+/-0.14, and 0.69+/-0.06 and μ_c=1.53+/- 0.03, 1.43+/-0.06, and 1.17+/-0.04. Radar albedo features are seen on each satellite. Evidence for μ_c features is lacking, except for indications of a weak hemispheric asymmetry for Callisto. That intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in μ_c is consistent with predictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance. The satellites' albedo distributions overlap. The most prominent radar features are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frequencies show Callisto to be lagging its ephemeris by 200+/-50 km.